PROJECT SUMMARY/ABSTRACT Metastatic lung cancer kills 160,000 people in the United States annually. In patients with early stage, surgically resected lung cancer 10-60% will develop recurrence and distant metastasis. There is an unmet need to predict the likelihood for developing distant metastases and for innovative adjuvant therapies that could decrease this risk. Our group has shown that the metastasis suppressor gene, Breast Cancer Metastasis Suppressor 1 (BRMS1), inhibits metastases in lung adenocarcinoma (LUAD). Two primary isoforms of BRMS1, v1 and v2 are present in humans. Next generation sequencing of BRMS1 reveals a single nucleotide polymorphism (SNP) rs1052566 (G>A) that causes an A273V mutation of BRMS1v2. The homozygous A allele (BRMS1v2A273V/A273V) is present in 8% of LUAD patients and correlates with a poor progression-free survival of patients with stage I-II, node-negative LUAD in the TCGA cohort (N=278). Mechanistically we show that BRMS1v2 A273V abolishes the metastasis suppressor function of BRMS1v2 and promotes robust cell invasion and metastases by activation of c-fos-mediated gene-specific transcriptional regulation. Specifically, BRMS1v2 A273V increases cell invasion in vitro and increased metastases in both tail-vein injection xenograft and LUAD patient-derived organoid (PDO) intracardiac injection metastasis in vivo models. Moreover, we show that BRMS1v2 A273V fails to interact with Src, resulting in c-fos activation and increased L1CAM. Inhibition of c-fos or knockdown of L1CAM reduces BRMS1v2 A273V-promoted cell invasion. Our overarching goals are 1) to clarify the contribution of the c-fos in BRMS1v2A273V/A273V LUAD metastases, and 2) to assess the efficacy of targeting BRMS1v2A273V/A273V to reduce metastases and improve survival. Two Specific Aims will test our hypotheses: Aim 1) Investigate the mechanisms through which BRMS1v2 A273V promotes LUAD metastasis. We will perform RNA-seq in our BRMS1v2WT/WT and isogenic BRMS1A273V/A273V cells followed with an in vivo CRISPR/sgRNAs screen to identify functional BRMS1v2 A273V downstream target genes and the requirement of c-fos. Next, we will express different forms of nuclear Src in LUAD cells to assess the role of nuclear Src/c- fos pathway in BRMS1v2 A273V-mediated gene-specific transcriptional regulation. Aim 2) Understand the significance of BRMS1v2A273V/A273V in promoting LUAD metastasis. We will assess the ability of BRMS1v2A273V/A273V to predict metastases by leveraging our extensive, clinically-annotated biorepository of 893 stage I-II (node-negative) human LUAD specimens. We then will use our newly developed PDO model to examine 1) the therapeutic significance of targeting c-fos in metastasis suppression of BRMS1v2A273V/A273V LUAD by repurposing the c-Fos inhibitor T5224, and 2) the dominant form of BRMS1 that governs metastases. Impact: Our work will provide mechanistic and translational evidence to highlight BRMS1v2A273V/A273V as an important germline marker to predict metastases in early-stage LUAD, and to provide justification for future clinical trials that target c-fos in BRMS1v2A273V/A273V LUAD to potentially decrease metastasis.